The invention concerns a method and a device for the power regulation of an underwater power plant with the features in the preamble of the independent claims.
Underwater power plants, which are free-standing in a surrounding flow and take up kinetic energy through a turbine to drive a directly coupled electric generator, are known and serve in particular to utilize tidal currents for production of electric energy. Usually a propeller-shaped water turbine, supported by a floating load-bearing structure or one placed on a foundation, is used to take up kinetic energy from the surrounding flow. For a simple design of such an underwater power plant, the water turbine is coupled directly to a gearing to convert the revolutions of the turbine into velocity. In both cases, there is a proportionality between the rotational speed of the water turbine and the number of revolutions of the electric generator, so that the revolutions of the water turbine, can be controlled by means or the electric generator.
For the power regulation of such an underwater power plant, the installed power must be maximized for slow to medium flow velocity. In one economical design of the plant, the power is limited above a given rated power.
Based on the above indicated regulation requirements, thus far a characteristic curve method of control has been used for a power-optimized plant operation, which starts with a measuring of the characteristic curve of the plant and assigns an optimal high-speed number to a particular flow velocity for which an optimal power coefficient and thus a maximum mechanical power uptake exists. This type of control system is contingent upon a sufficiently accurate measurement of the oncoming flow characteristic, besides the characteristic curve of the plant, taken as known. This requires a separate measurement system for the flow velocity, which is a system component burdened with a certain likelihood of malfunction. Furthermore, a measurement of the oncoming flow acting on the water turbine is problematical, due to local flow differences. Moreover, one must consider that the flow measurement system, while needing to be placed sufficiently close to the water turbine in order to receive the same flow as the water turbine, also needs to maintain such distance from the water turbine that the flow around the latter does not significantly affect the measurement of the field of flow.
A further drawback to a method based on characteristic curves is that, in the course of the aging of the plant, for example, due to algal growth or wearing of mechanical components, especially the bearing of the revolving unit, a deviation can occur from the originally measured characteristic curves forming the foundation of the control system.
To remedy the aforementioned problems, a refinement of a characteristic-based assigning of nominal rpm values for water turbines by a search algorithm for the power optimum has been proposed for hydraulically operated machine sets. We refer to DE 3601289 A1 and EP 0230636 B1 for this. To find the power maximum, a time variation for the manipulated variable is switched onto the speed control and the resulting power fluctuations are analyzed. Incremental correction values are switched into the nominal speed value consecutively so that an optimal effective power is put out to an electricity network by the machine set. Such a regulator is known as a maximum power point regulator (MPP regulator).
The drawback to the use of a MPP regulator for speed control is that fluctuations in the oncoming flow velocity in particular become superimposed as a perturbation on the systematically caused power variations of the search process that the power optimization is impaired. Moreover, when oncoming flow conditions are fluctuating heavily, which can occur at a tidal power plant during rough seas and turbulence in the current, the speed control is a detriment to the power regulation, since the adapting of the rpms and power often is not fast enough, and therefore the underwater power plant is subjected to a high torque input, which can only partly be used for producing electric energy.
A blade angle adjustment can be done to change the characteristic curve of the plant in order to accomplish the power regulation subject to a limiting of the power input. If, instead, the rotor blades of the water turbine are provided with a fixed installation angle, for the power-limited operating range one can slow down or speed up the water turbine to approach a working point outside of the power optimum. Sufficiently precise determination of the actual flow onto the water turbine is especially critical for this power-limited operating range.